The present invention relates generally to photoresist processing systems, and more particularly to a thermal cycling module for subjecting a substrate to a controlled temperature cycle.
To photolithographically fabricate an integrated circuit, a substrate, such as a semiconductor wafer, is coated with a layer of photoresist. The photoresist layer is exposed and then chemically developed to define circuitry features. As part of a photoresist processing step, the substrate may be subjected to a controlled thermal cycle to set or harden the photoresist layer. Typically, the substrate is heated to an elevated temperature, e.g., 70xc2x0 to 250xc2x0 C., maintained at the elevated temperature for a preselected duration, e.g., 30 to 120 seconds, and then cooled to a reduced temperature, e.g., 0xc2x0 to 30xc2x0 C. The temperature of the substrate must be precisely controlled during this thermal cycle to achieve a high yield.
Referring to FIG. 1, in a conventional photoresist processing system 20, a substrate 10 is initially placed on a hot plate 22 where it is heated. The substrate is then mechanically transported to a cold plate 24 where it is cooled. There are several disadvantages with this method of thermal cycling. First, the movement of the substrate through the environment from hot plate 22 to cold plate 24 subjects the substrate to uncontrolled and non-uniform temperature fluctuations. Second, temperature non-uniformities may arise from convection currents produced inside the plates during heating and cooling. Third, the time required to transport the substrate between the plates reduces the throughput of the system. Fourth, the substrate may be contaminated by the mechanical transport.
Referring to FIG. 2, in a prior art photoresist processing system 30, described in U.S. Pat. No. 5,431,700 and incorporated herein by reference, one of the plates, e.g., hot plate 32 is placed upside down and directly above the other plate, e.g., cold plate 34. The substrate 10 is suspended, e.g., with a lift pin assembly, between the two plates. Because substrate 10 only moves a short distance between hot plate 32 and cold plate 34, system throughput is improved and non-uniform temperature fluctuations experienced by the substrate are reduced. Nevertheless, because the substrate is moved between the plates, it can still experience uncontrolled and non-uniform temperature fluctuations and is subject to possible contamination.
Referring to FIG. 3, another prior art photoresist processing system 40, described by PCT Patent Publication WO9805060 and incorporated herein by reference, includes a single bake/chill plate 42. A passage 44 is formed through the body of plate 42. To raise the temperature of substrate 10, a hot fluid (e.g., between 150xc2x0 and 250xc2x0 C.) from a hot fluid supply 46 is cycled through passage 44 via a pipe or conduit 45. Similarly, in order to lower the temperature of the substrate, cold fluid from a cold fluid supply 48 is cycled through passage 44. The photoresist processing system 40 may also include an array of thermoelectric devices 49, such as resistive heating elements, positioned between the bake/chill plate and the substrate. Each thermoelectric device may be individually controlled by an associated temperature sensor and feedback circuit to ensure a uniform temperature across the substrate surface.
Although the substrate need not be mechanically transported between different plates in photoresist processing system 40, there are significant disadvantages associated with this system. First, the temperature of the entire plate 42 must be adjusted to control the temperature of the substrate. Since bake/chill plate 42 is a large thermal mass, its temperature can not be changed quickly, thereby reducing the throughput of the system. Second, due to the large thermal mass of bake/chill plate 42, operation of the photoresist processing system consumes a large amount of power. Third, the fluid supplies 46 and 48 are bulky and expensive. Fourth, the hot fluid flowing through pipe 45 posses a safety threat to personnel working near the system.
In one aspect, the invention is directed to a thermal cycling module for thermally processing a substrate. The thermal cycling module includes a thermally-conductive support structure having a first side in thermal contact with the substrate during processing, a heater to increase the temperature of the substrate when the substrate is in thermal contact with the first side of the support structure, and a fluid distributor to direct a coolant onto a second side of the support structure to decrease the temperature of the substrate.
Implementations of the invention may include the following. The first side of the support structure may be substantially planar, and the second side of the support structure may include a plurality of recesses extending toward, e.g., almost entirely through the support structure, the first side. The distance between the bottom of the recesses and the first side may be about 20 mils. The fluid distributor may directs at least a portion of the coolant into the plurality of recesses. The recesses may be generally cylindrical in shape and may be disposed in a generally hexagonal array, and the support structure may include a plurality of support struts located between adjacent recesses. The heater may be a resistive heater deposited and bonded on the first side of the support structure. The heater may include a plurality of heating zones, and the thermal cycling module may further comprise a controller to independently control the temperature of each heating zone. The fluid distributor may include a plurality of nozzles to direct a spray of coolant, e.g., water, onto the second side of the support structure, e.g., with a substantially uniform flow. The fluid distributor may include a rotatable arm, and the nozzles may be attached to the arm and fluidly connected via a passage in the arm to a coolant supply. The arm may be rotated by the flow of fluid through angled nozzles, or by a motor. A housing may be secured to the support structure to form a fluid-tight chamber, and the fluid distributor may be is located in the chamber. The fluid distributor may include a ring-shaped member positioned below a plurality of lift pin assemblies, and an actuator may vertically move the fluid distributor such that the ring shaped member can contact the lift pins to cause movement thereof.
In another aspect, the thermal cycling module includes a support structure having a first surface adjacent to which the substrate may be positioned during processing, a heater to increase the temperature of the first surface to increase the temperature of the substrate, and a fluid distributor to direct a coolant onto a second surface of the support structure to lower the temperature of the first surface to thereby lower the temperature of a substrate positioned adjacent thereto.
In another aspect, the thermal cycling module includes a bake-chill plate and a fluid distributor. The bake-chill plate includes a resistive heater to raise the temperature of the substrate during processing and a plurality of recesses in a first surface thereof, and the fluid distributor directs a coolant spray at the first surface and into the recesses of the support structure to lower the temperature of the substrate.
In another aspect, the thermal cycling module has a bake-chill plate including a resistive heater to raise the temperature of the substrate and a honeycomb support structure to support the heater, and a fluid distributor to direct a coolant spray at an underside of the support structure to lower the temperature of the substrate.
In another aspect, the invention is directed to a method of thermally processing a substrate. In the method, a substrate is supported on a first side of a support structure, the temperature of the substrate is raised to an elevated temperature, the substrate is maintained at the elevated temperature for a predetermined period of time, and a coolant is directed onto second side of the support structure to lower the temperature of the substrate.
Advantages of the invention may include the following. The thermal cycling module can precisely control the temperature of a substrate, and can heat and cool the substrate using less power. The substrate remains positioned on a single bake/chill plate, thereby reducing the danger of contamination and non-uniform temperature fluctuations. Furthermore, the system is safer because the temperature cycling module does not require a hot fluid.
Other advantages and features of the invention will become apparent from the following description, including the drawings and claims.